The present invention relates generally to motor vehicle emissions testing and, more particularly, to a computer controlled sealed housing evaporative determination (SHED) testing apparatus and method for automatically testing motor vehicle evaporative emissions for practically zero emission vehicles (PZEV) under a number of testing schedules.
In addition to commonly known tailpipe exhaust emissions produced during engine operation, there are also evaporative emissions which are generated while the vehicle is simply sitting parked. Testing for evaporative emissions is typically conducted according to what is commonly known as a sealed housing evaporative determination (SHED) test.
Modern regulations require measuring evaporative emissions produced by a vehicle over the course of several days and in response to changing temperature conditions. However, changes in air temperature cause corresponding changes in the volume and hence changes in the pressure of the ambient air within the SHED structure. Pressure differences between the interior and the ambient air of the SHED structure and the outside environment encourage migration of air either into or out of the testing structure through any leaks, thus affecting the accuracy of the test results. Therefore, it is desirable to control the pressure changes in order to maintain a pressure difference between the interior of the SHED structure and the surrounding outside atmosphere as near zero as possible.
One known approach to performing variable temperature SHED tests while controlling the pressure differential is set forth in U.S. Pat. No. 5,592,372. While the apparatus and methods disclosed by the '372 patent are suitable for emission testing on standard vehicles, new approaches are now required for PZEV testing, because the levels of hydrocarbons being monitored are much lower than those of standard vehicles. PZEV testing additionally requires a breakdown of the components of the hydrocarbons being emitted to determine the source of the emissions—e.g. refrigerant from the air conditioning system versus fuel vapor from the fueling system.
Hence, there is seen to be a need for SHED testing of PZEV's wherein the evaporative emission levels being monitored are substantially lower than for standard vehicles tested under previous arrangements.